But when Gracely determined the proper vibrations of his oscillating current to coincide with all the other material factors he was using, the final result was before him-real invisibility. He used a patterned background--a symmetrically checkered surface, most difficult of all. The light rays coming from this background passed through the magnetic field surrounding the invisible colorless cube, and were bent into a curved path. But their own color-spectrum--in actuality the color, shape, all the visible characteristics of the background--was not greatly altered. The observer saw what actually was behind the invisible cube: the checkered background, sometimes slightly distorted, but nevertheless sufficiently clear for its abnormality to escape notice. Thus the cube's outlines were not reconstructed; and, in effect, it had vanished. In practical workings with the X-flyers, no such difficult test as Gracely's cube and rectangular, symmetrically patterned background is ever met. The varying background behind a plane--at rest or flying, and particularly at night--demands less perfection of background than Gracely's laboratory conditions. I am informed that an X-flyer can vaguely be seen--or sensed, rather--from some angles and under certain and unfavorable conditions of light, and depending on its line of movement relative to the angle of observation, and the type and color-lighting of its background. But under most conditions it represents a very nearly perfect mechanical invisibility. There is one aspect of the subject with which I may close this brief paper. I give it without technical explanation; it seems to me an amusing angle. The theory of stereoscopics--the vision of the twin lenses of the human eyes, set a distance apart to give the perception of depth, of the third dimension--is in itself a subject tremendously interesting, and worthy of anyone's study. I have no space for it here, nor would it be strictly relevant. I need only state that a two-eyed man sees partially around an object (by virtue of the different angles from which each of his eyes gaze at it) and thus sees a trifle more of the background than would otherwise be the case. And this--these two viewpoints blended in his brain--gives him his perception of "depth," of "solidity"--the difference between a real scene of three dimensions and a painted scene on a canvas of two dimensions with only the artist's skill in perspective to simulate the third. And I cannot refrain from mentioning that in Government tests of the Anti-War Department to determine the perfection of the invisibility of the X-flyers, it was a one-eyed man who proved that they were not always totally invisible!--Ray Cummings. _Thank You_ Dear Editor: